1. Field of the Invention
This invention relates to an optical inspecting apparatus for two-dimensionally or three-dimensionally viewing a surface of a sample or the like, and more particularly to an optical inspecting apparatus of this kind which is suitable, e.g. for inspecting the state or condition of a surface (hereinafter referred to as a "sample surface") of a sample, such as a mirror-finished surface wafer, or for detecting a symbol, particularly an ID (identification) number (serial number as an identifier), formed on the sample surface.
2. Prior Art
Conventionally, an optical inspecting apparatus is used for checking the state of the sample surface for an undulation, a dimple, a projection, insufficient wash, or buff damage, or for detecting an ID number formed on a sample surface of a member used for manufacturing semiconductor integrated circuits, etc. The present applicant has already proposed an inspecting apparatus of this kind, which comprises (1) an illuminating optical system for illuminating a sample with light, (2) a schlieren optical system which includes an optical element for focusing light transmitted through the sample or light reflected from the sample, and has an aperture stop arranged on a back focal plane of the optical element or in its vicinity in an image space of same, and (3) a viewing block for viewing light having passed through the aperture stop (Japanese Provisional Patent Publication (Kokai) No. 6-3625).
The schlieren optical system used in the inspecting apparatus is one of typical optical systems for indicating variations in refractive index, reflectance, and transmittance resulting from the irregularities on the sample surface or the internal condition of the sample, as differences in brightness. This optical system is adapted to cause the light reflected from the sample surface to be focused by the collimator lens, and a knife edge arranged on a back focal plane of the collimator lens in an image space thereof intercepts scattered portions of the light reflected from the sample surface, permitting an image on the air of the sample surface, which is formed by the collimator lens in the air behind the collimator lens, to be viewed by the eye or by the use of a camera or other devices (a viewing block).
The term "image on the air" means a focused image formed in the air or on an imaginary plane perpendicular to an optical axis of a lens or lens system, at a predetermined location in its image space, which would be visible on a screen placed at the predetermined location. The predetermined location depends on the characteristics of the lens of the first telecentric system 17.
According to this optical system, if there is an irregularity on the sample surface, the light is scattered from the irregularity. Part of the scattered light from the irregularity hits against the knife edge, and is prevented from reaching the viewing block, As a result, when viewed from behind the knife edge, the portion (irregularity) of the sample surface corresponding to the scattered light intercepted by the knife edge becomes darker than the rest of the sample surface. A pattern of bright portions and dark portions corresponds to the state of the sample surface, and hence it is possible to judge the slate of the sample surface from this pattern.
However, these conventional optical inspecting apparatuses, including the proposed one, uniformly have the viewing block arranged in a location immediately behind the knife edge, where an image on the air is formed. Therefore, the position of the viewing block is not necessarily convenient to a viewer who views the image. Further, when a plurality of viewing blocks having different magnifications are to be provided, the viewing blocks are provided close to each other at the location where the image on the air is formed, so that the freedom of arrangement of these viewing blocks is very limited.